The present disclosure is directed to both a method and apparatus for providing data both and dynamically during the drilling process. This can be used with drilling apparatus which is drilling an oil or gas well including wells which are vertically drilled or highly deviated wells. It can be used with conventional surface powered drilling systems meaning those which incorporate the traditional drilling rig providing power to the drill stem through a rotary table (via a kelly) or it can also be used with similar drilling equipment using a mud motor in the drill stem located just above the drill bit. It is a device and system which provides the weight on the bit (sometimes WOB hereinafter) and other forces acting on the drill stem in the vicinity of the drill bit. The present apparatus is described in the context of a drill stem which is powered from the surface by a typical rotary drilling rig. The equipment, however, further includes and incorporates a mud motor cooperating with a bent sub of the sort normally used in drilling a deviated well so that the variety of circumstances in which the WOB and other forces are measured can be exemplified.
Consider a typical well to be drilled, perhaps one of average depth. Wells drilled in Texas have averaged about 12,000 feet in depth. A well of that depth involves the assembly of a drill stem which is made up of a string of drill pipe. The drill pipe is connected to one or more drill collars and then in turn connect with a drill bit which is placed at the bottom of the drill stem. Historically, the vast majority of continental drilling has been vertical. This is no longer true. Presently, a significant percentage of continental wells are highly deviated or even horizontal. Deviated onshore drilling has gained in popularity in order to take advantage of natural fracture systems which act as a network of fluid conduits in some hydrocarbon bearing reservoirs. These natural fracture systems, if properly intersected with a deviated directional boreholes, can increase the probability of a producing wells and also increase the production rate of producing wells. In many areas of the world, offshore wells are also drilled directionally to take advantage of naturally occurring fractured zones. Even in unconsolidated target formations such as many producing zones in the Gulf of Mexico, offshore drilling involves deviated wells so that several wells can be brought to a single wellhead location type well at an offshore platform. Both onshore and offshore, directional drilling is of great commercial importance in the present day exploration and production of hydrocarbons.
Drilling efficiency can be greatly improved if the driller has real time access to a measure of the force components acting upon the drill string. The present invention is directed toward improving drilling operations and more specifically improving the efficiency of directional drilling operations by measuring axial force, torque, bending moment and shear force components acting upon the drill string in the vicinity of the drill bit. These measurements are suitable for transmission to the surface of the earth in real time. The invention contemplates the use of a cylindrical subsection of the drill string, or "sub", which is placed in the drill string immediately above the drill bit to measure strains resulting from the plurality of forces acting in the vicinity of the drill bit. In particular, the WOB is one parameter which is important to the control of the drilling process. The weight substantially impacts the performance of the drilling process. For instance, the weight must be controlled so that the drill bit is provided with weight in a selected range for proper operation. If the weight is excessive, it tends to damage the drill bit. If the weight is below the desired range, the rate of penetration might be reduced. The bite of the drill bit into the formation is dependent on a number of variables including the speed of rotation, the type of drill bit, the physical characteristics of the formation being penetrated, and most especially the weight dynamically experienced at the drill bit during the drilling process. It is therefore important to measure WOB. A knowledge of other forces experienced by the drill string in the vicinity of the drill bit is likewise important to the driller. Torque is probably the most important parameter when diamond or PDC type drill bits are employed. As an example, an abnormally large torque may indicate excessive bit wear or that the WOB is improper for the formation being penetrated. The summation of oscillatory plus mean torque should be kept positive to prevent tensile stress in the PDC elements that would cause their fracture. Instances where the drill bit is literally supported by the fluid pressure at a level above the bottom of the hole, which is known as "pump off", is best detected by a measure of torque. Excessive shear forces may indicate that a salt formation is being penetrated and that the drilling fluid composition is not proper for this phase of the drilling program. An excessive pressure indication usually indicates that a high pressure zone is being penetrated and may indicate the need to adjust the weight of the drilling fluid to compensate for the increase in downhole pressure. Finally, unexpected bending forces may be indicative that the deviated drilling program is not proceeding according to plan.
The present system recognizes that all forces acting upon the drill string are dynamic and vary depending upon the circumstances. In the most simple case, the WOB is substantially a function of the depth of the drill bit in the borehole when the well is substantially vertical, assuming that there is little snagging of the drill string in the borehole and there is minimal variation in the drill bit as it rides on the face of the drilled well borehole. In practice a portion of the drill string weight is reacted by the drilling rig draw works in order to keep the WOB at its optimum value. The draw works reactance force is adjusted using the weight of the drill string in the borehole and the buoyancy of the drill string. In theory, drill string weight can be estimated rather accurately by knowing the drill pipe schedule. Specific weights are known for the drill collars, drill pipe and respective lengths thereof. If ten drill collars are incorporated totaling 300 feet in length, the weight attributable to the drill collars can be quickly determined. Likewise, the weight of the drill string can be determined quite easily. Drill string buoyancy can also be estimated using the density of the drilling fluid. Under the most ideal conditions, the WOB can, in principle, be determined and controlled by adjusting the reactance of the draw works as a function of the weight of the drill string in the borehole and the drill string buoyancy. While that simplified circumstance is common, it does not always provide the correct answer. The actual WOB will vary for a variety of conditions and circumstances as will other forces acting on the drill string.
One instance in which the WOB varies significantly from the weight of the drill stem is when the well is deviated from the vertical. At this juncture, the well is no longer a simplified vertical hole in which all the weight of the drill stem rests on the drill bit. Rather, the drill stem in some part will be on its side. This reduces the WOB. Even with wells that are more or less vertical, it is possible that some of the weight is held up by contact between the opened hole and the drill string uphole from the drill bit. As an example, modest deviations that give rise to the sticking of the drill pipe or "key slotting" may create an interference in the open well borehole which holds up the drill string and supports a part of the weight, thereby reducing the actual WOB.
Another aspect of drilling a deviated well arises from the situation in which the WOB is distorted with rotation. This provides a modulation of the WOB as measurements are made dependent on movement of the measuring device from the low side to the high side of the deviated borehole. The present disclosures sets forth a method and apparatus for dynamically measuring the axial force indicative of WOB, as well as other forces, by utilizing multiple sensors arranged around a drill string sub located just above the drill bit. The undulations provide a modulated variation on the signal so that the deviated well modulations can be recognized, and the average value of WOB as well as other forces can determined. As mentioned previously, other types of distortion in the process may occur. For instance, the WOB as sensed just above the drill bit may vary with ambient pressure in the drill stem. Recall that drilling fluid or "mud" is normally pumped under pressure from the surface through the drill stem to flow out of the drill bit and return to the surface by way of the annulus between the drill string and the borehole wall. The drilling mud removes cuttings, lubricates the bit, cools the bit and serves to balance the pressure of the formations penetrated.
In another aspect, the weight on bit may also be altered, as the drill bit rotates, as a result of shear stresses created. If the drill bit is dynamically pushed to the side, so to speak, a shear force at right angles to the axis of the drill stem is created. The shear forces on the bit and the bending stress in the stem are direct measures of steering deviations. These factors can be recognized and controlled in the drilling program to provide the desired path of the borehole.
In summary, a variety of difficulties can arise in the drilling of a well. Many of these difficulties can not be predicted yet they greatly affect the efficiency and even the safety of the drilling operation. Measures of forces acting upon the drill string in the vicinity of the drill bit, if made while drilling, can be used to maximize drilling efficiency. The present disclosure thus sets forth a means and mechanism for measuring strains resulting from these forces.